It has long been recognized that many of the waste materials generated by human society can, ultimately, be broken down into a small number of simple organic materials that have their own intrinsic value. The ability to implement such transformation in an energy-efficient manner and on a large enough scale would be of tremendous benefit to society.
Most living materials, as well as most synthetic organic substances used in domestic and commercial applications comprise carbon-based polymers of various compositions. Under appropriate conditions, most such materials—including wood, coal, plastics, tires, and animal waste—will break down to a mixture of gaseous products, oils, and carbon. Materials such as agricultural waste materials may also contain inorganic substances that break down to mineral products. Almost all of these products, whether organic or inorganic, can enjoy new lives in a host of beneficial and often lucrative applications.
Not only is the principle of creating useful materials from otherwise unserviceable waste appealing: recycling of waste materials is of fundamental importance to the way that the burgeoning human population will come to cope with major challenges in the 21st century. Two principal challenges facing humanity are coping with a finite supply of materials and energy, and with curtailing the growing threat to the environment from global warming. Indeed, an idea that is rapidly gaining currency is that recycling carbon-based materials from within the biosphere rather than introducing new sources of carbon from underground oil, natural gas and coal deposits could mitigate global warming.
As of today, however, industries that produce huge volumes of waste materials comprising largely organic materials face enormous challenges in disposing and storing that waste, as well as putting it to maximum beneficial use.
A case in point, the food processing industry around the world generates billions of pounds of organically rich wastes per year. These wastes are associated with the processing of both animal and plant products, and include turkey-, fish-, chicken-, pig-, and cattle-processing and husbandry wastes. The food processing industry continues to grow and its members face significant economic and environmental pressures to do something productive with their waste materials. Such waste materials give rise to a number of critical problems. The generation of greenhouse gases such as carbon dioxide and methane by landfilling, land applying, or digesting food wastes, without any other benefit, is one such problem. Ideally, the food industry must adopt efficient and economical ways of managing their wastes without discharging odorous or objectionable pollutants.
More recently, the cost of warehousing unusable byproducts in many areas is growing in significance. As the types of waste materials that can be fed to agricultural livestock become increasingly regulated. For example, in the wake of BSE/CJD scares in Europe, many waste materials are simply being warehoused, pending a suitable fate. Clearly, there is an additional urgent need to find an acceptable means to cleanly process and utilize such materials. Preferably, a way to convert food-processing wastes into useful, high-value products needs to be found.
An additional drive to seek treatment alternatives is the combined enforcement of wastewater discharge regulations and the escalation of sewage surcharges. The food processing industry must seek cost-effective technologies to provide pretreatment or complete treatment of their wastewaters and solid (wet) wastes. Historically, food processing facilities located within or adjacent to municipalities, have relied on local publicly owned treatment works (POTWs) for wastewater treatment and disposal. Increasingly, this option is becoming less available, as a result of more rigorous enforcement. Pressure to comply with wastewater discharge permits has increased. Dwindling federal grants for construction of new and upgraded POTWs also mean that this option is less appealing. Thus, the food-processing industry is increasingly being pressured to devise more effective ways of disposing of its inedible products.
Bioaccumulation of persistent chemicals such as dioxins and the potential for the spread of life threatening diseases such as Mad Cow Disease (BSE) is another threat to food processors and food consumers alike. This threat is greatly exacerbated by refeeding food processing residues to farm animals. Food processors need economical solutions to break this cycle.
Treatment of industrial waste, namely shredder residue, likewise presents another challenge. While most components of end-of-life automobiles, household and commercial appliances can be recycled, reused, or recovered, a significant portion is left over from the shredding process and finds its way into landfill. Disposal of shredder residue is made all the more difficult by the toxic materials found therein, e.g. cadmium, lead, mercury, and other heavy metals. Due to the limited amount of space available for landfill use and the increasing costs of hazardous waste disposal, an alternate solution is needed. The automotive and recycling industries are currently under pressure to devise ways of using shredder residue in a cost-effective and energy-efficient manner.
Furthermore, municipal and regional sewer authorities are requiring industries to reduce their organic biochemical oxygen demand (BOD), chemical oxygen demand (COD), and solid loading on the sewers. Due to the high BOD concentrations typically found in high-strength food process wastewaters with high levels of suspended solids, ammonia, and protein compounds, the food processing industry is under additional scrutiny. Food processing facilities need cost-effective and application-specific treatment technologies to manage their wastewaters and solid wastes effectively.
Similar problems are multiplied, magnified and augmented in many different ways across other industries. For example, the generation of malodorous air emissions associated with rendering plants—that convert animal waste by heat into fats and proteins, is one such problem. Another is land application of municipal biosolids that contain high concentrations of pathogens.
There have been various approaches developed to process used waste tires, say from truck and passenger vehicles, into useful products including fuels, petroleum oils, carbon, fuel-gases, and feedstocks for manufacture of tires and other rubber products. Typically, these schemes involve heating and dissolving the tires in solvents. Some of the schemes attempt to devulcanize the tire rubber, i.e., break the sulfur bonds that connect the constituent polymers along their lengths. Others attempt to depolymerize the rubber material. Depolymerization breaks the long chain polymers into a composition of smaller subunits with higher fluidity and greater utility, such as fuel oil. Some schemes involve using water under conditions near or above its critical point (˜3,200 psi and ˜370° C.) at which water can be an effective solvent for and reactant with the tire feedstock. However, such schemes are rendered energy-inefficient by virtue of the amount of energy needed to achieve super-critical conditions. Processing at super-critical conditions is also not cost-effective as it requires expensive super-alloy operating equipment.
A number of organic materials have been considered for dissolving tire material to form a heavy oil or a devulcanized rubber product. Existing schemes that operate at modest conditions (<200 psi) generally produce heavy, contaminated products, whereas those that use lighter solvents produce better products but also require a more expensive solvent or higher operating pressure (>2,000 psi), or both. Additionally, most schemes that use a solvent to dissolve tire material are uneconomical due to loss of some fraction of the solvent during the process and the cost associated with the make-up solvent, even in instances where solvent recovery and reuse can be practiced.
Aerobic and anaerobic digesters have been employed at sewage treatment plants to treat municipal sewage sludge. There are a number of problems associated with their use. The basic principle behind their operation is that biologically rich materials are directed into large holding vessels that contain bacteria which digest the biological materials. Typically, dissolved solids are directed to an aerobic digester, and suspended solids are directed to an anaerobic digester. Once the nutritional feed materials are exhausted, the bugs can no longer sustain themselves, and they die. The end-product of the digestion period is a sludge that contains the dead bacteria, and which must be disposed of in some way. One problem with the resulting material is that it still contains pathogens. Problems with the whole process, in general, include that the holding times in the digester vessels can be as long as 17 days, and that the operating conditions are difficult to maintain. For example, the relatively large vessel (typically 20-30 ft. in diameter) is usually maintained at above 85° F., and in some cases above 122° F.
All of the disposal technologies currently available to industries, in particular the food processing industry, have significant limitations and drawbacks that provide an incentive to search for alternative processes. This applies to technologies in addition to the use of existing POTWs. In particular, four types of approach, land disposal (landfills, composting, land application), biotreatment, traditional thermal oxidation treatments such as incineration/combustion, and pyrolysis/gasification, all have separate drawbacks.
Drawbacks for land disposal include: high haulage or transport costs, significant potential for groundwater contamination from leaching, and the exposure of area residents to high concentrations of hazardous pollutants (such as pathogens in the instance of land application). Landfills produce gas that can create air pollution concerns, including the generation of greenhouse gases.
Biotreatment of waste also has its disadvantages. The process is difficult to control and its performance equally difficult to verify. How well the process performs largely depends on whether adequate airflow, i.e. oxygenating means, eis provided to the soil where aerobic bacteria is involved. Additionally, bacteria that may have been developed to consume specific compounds will, when placed in soil, activate alternative enzyme systems to consume the easiest available compounds.
Drawbacks associated with older incineration or combustion units include the need to add equipment or components to meet increasingly heightened air pollution emission standards. It may also take longer to obtain air discharge permits for incinerators than for other technologies due to significant community concerns about incineration. Additionally, the treatment of the waste at the exhaust means treating large volumes of gas so that very large plant equipment is required. The feedstock is also low in calorific value. Some incinerators are not compatible with solid fuels or solid waste, as these materials will start to oxidize too high up in the furnace. Conversely, high moisture content in the feedstocks is also a problem because during incineration or combustion the water is vaporized and removed—a process which requires approximately 1,000 Btu/lb of water vaporized. This represents huge heat/energy losses to the system.
The last category of technique employed—pyrolysis/gasification—is appealing because, unlike the others mentioned, it attempts to convert the waste into utilizable materials, such as oils and carbon. Of principal concern in implementing ways to break down waste ematerials is finding a means of controlling the composition of the resulting products while minimizing the amount of energy needed to effect the breakdown. Generally, pyrolysis and gasification methods employed in the past were aimed at breaking down the waste materials in a single stage process, but a single stage has been found to offer inadequate control over purity and composition of the end products.
Pyrolyzers have been used to break down organic materials to gas, oils and tar, and carbonaceous materials. A pyrolyzer permits heating of the organic materials to high temperatures, ˜400-500° C., but has poor energy efficiency and gives little control over the composition of the resulting materials. In particular, most waste materials—especially those originating from agricultural sources—contain up to 50% water. To effectuate the breakdown, pyrolyzers in the art would boil off the water using a very energy-demanding process. Additionally, a pyrolysis chamber is typically large in size so as to maximize throughput. However, use of a large chamber also has the unfortunate side-effects of generating significant temperature gradients throughout the chamber, resulting in uneven heating of waste materials and poor quality or impure end products.
Gasifiers have been used to achieve a partial combustion of waste materials. In essence, a gas—usually air, oxygen, or steam—is passed over the waste materials in an amount that is insufficient to oxidize all the combustible material. Thus, some combustion products such as CO2, H2O, CO, H2 and light hydrocarbons are produced, and the generated heat converts the remaining waste materials into oils, gases, and carbonaceous material. The gases produced will contain some of the input gases, but any gases that are produced are too voluminous to be stored and must be used immediately or piped to a location where they can be utilized. Gasifiers also suffer from some of the same drawbacks as pyrolyzers, e.g. high energy consumption in vaporizing water content of waste material.
Products of pyrolysis and gasification methods also tend to contain unacceptably high levels of impurities. In particular, sulfur- and chlorine-containing materials in waste materials give rise, respectively, to sulfur-containing compounds such as mercaptans, and organic chlorides in the resulting end products. Typically, chlorinated hydrocarbons at levels of 1-2 ppm can be tolerated in hydrocarbon oils, but neither gasification nor pyrolysis methods can guarantee such a low level with any reliability.
Furthermore, pyrolysis and gasification methods have low efficiencies, typically around 30%. One reason for this is that the products are not optimum in terms of calorific content. Another reason is that a single stage process cannot readily produce materials in a form from which energy can be efficiently harnessed and recycled in the process. For example, it is difficult to capture the thermal energy in the solid products that are produced and redirect it to assist in the heating of the reaction vessel.
As detailed above, pyrolysis/gasification methods suffer in several ways. The oil product is generally rich in undesirable high viscosity components such as tar and asphalt. Both pyrolysis and gasification processes have poor heat transfer properties and consequently do not heat evenly. Therefore, end products vary greatly in number with few of sufficient quantity or quality for economical recovery. Wet feedstocks require significant energy to vaporize and represent large energy losses to the system since the water leaves as a gas in the stack. In summary, pyrolysis/gasification has a high overall operating cost, is capital intensive, and produces some by-products of no or limited value.
Although there have been many variants of the pyrolysis and gasification methods, all of which have suffered from broadly similar drawbacks, one recent advance has permitted significant increases in processing efficiency. For example, U.S. Pat. Nos. 5,269,947, 5,360,553, and 5,543,061, disclose systems that replace the single-stage process of the prior methods with a two-stage process. In a Hydrolysis Stage (often referred to as the “wet” stage), the waste materials are subjected to heat at around 200-250° C. and at about 20-120 atmospheres pressure. In preferred embodiments, the waste materials are subjected to a pressure of about 50 atmospheres. Under such conditions, the water content of the waste material hydrolyzes many of the biopolymers such as fats and proteins that may be present to form a mixture of oils. In a second stage (often called the “dry” stage), the mixture is flashed down to low pressure, during which around half of the water is driven off as steam. The mixture is heated still further to evaporate off the remaining water while the mixture ultimately breaks down into gaseous products, oils, and carbon.
The principal advance of these two-stage methods was to permit generation of higher quality and more useful mixtures of oils than any previous single stage process. However, the products of such methods still suffer from problems of contamination, from materials such as sulfur- and chlorine-containing compounds, and the need to evaporate a significant portion of the water still entails a substantial energy penalty. Thus, prior two stage methods have been difficult to make commercially viable.
Accordingly, there is a need for a method of processing waste and low-value products to produce useful materials in reliable purities and compositions, at acceptable capital and operational cost.